The present invention relates to a fuel cell for operation at normal temperature, which is used in portable power sources, power sources of distribution type, power sources for electric vehicles, power source systems for home and cogeneration systems and the like.
A fuel cell provides electricity and heat at the same time by electrochemically reacting a fuel gas such as hydrogen with an oxidant gas such as air in catalytic layers of electrodes. As an electrolyte for disposing between the electrodes, SiC matrix material impregnated with phosphoric acid is used in a phosphoric type fuel cell. In a polymer electrolyte fuel cell, perfluorocarbon sulfonic acid membrane is used. On both surfaces of the electrolyte, catalytic reaction layers (hereinafter simply referred to as xe2x80x9ccatalytic layersxe2x80x9d) of the electrodes, which are mainly composed of a carbon powder with a platinum type metallic catalyst carried thereon, are formed in close adhesion. Further, on the outer surfaces of the catalytic reaction layers, a pair of electrode base materials having gas permeability and electrical conductivity are placed in close adhesion. These base materials and the catalytic layers constitute the electrodes. This assembly of the electrodes and the electrolyte is called membrane electrode assembly (MEA). Moreover, on the outside of the electrodes, electrically conductive separator plates for mechanically fixing the MEAs and electrically connecting in series adjacent MEAs with each other are placed.
Generally, carbon fiber is used for the electrode base material and carbon is used for the separator plate. In the portion where the separator plate is in contact with the electrode, a gas flow path for supplying a reaction gas to the electrode surface and for exhausting a generated gas and an excess gas is formed.
At the electrode into which hydrogen is supplied, hydrogen, which is supplied from the gas flow path to the catalytic layer through the electrode base material, is oxidized and transformed into a hydrogen ion to come into the phosphoric acid solution. On the other electrode in which air is supplied, oxygen is reacted with a hydrogen ion in the phosphoric acid solution to generate water. As a result, an electron flows from the hydrogen side electrode to the air side electrode through the outer circuit to generate electric power. Accordingly, it is necessary to secure a route for supplying reaction gases such as hydrogen and air to respective catalytic layers and for removing drain gases such as water vapor.
In a solid polymer electrolyte fuel cell, for example, reaction gases are supplied after moistened in order to maintain the water-containing state of the polymer electrolyte membrane to saturation. Meanwhile, a reaction-generated water generates along with the electric generation reaction, and the reaction-generated water is added to the reaction gases supplied after moistened. As a result, the concentration of the reaction gases lowers with water vapor and, therefore, it is necessary to supply a large quantity of gases to the reaction sites on the electrodes and inside the catalytic layers in order to realize a high output.
For such a reason, water-repelling treatment has conventionally been carried out by applying a water-repelling agent such as a fluoric polymer, e.g. polytetrafluoroethylene on the catalytic layers, the electrode base materials, and the surfaces of the gas flow paths on the separator plates. It is considered that such water-repelling agent has a role of preventing the phosphoric acid solution impregnated in SiC matrix from leaking outside the cell.
The water-repelling agent such as a fluoric polymer has conventionally been applied to a predetermined portion in the following manner. For example, a colloidal dispersion of a fluoric polymer is impregnated and applied to the carbon fiber papers and the gas flow paths on the separators and, then, the solvent is removed by drying. Subsequently, by the thermal treatment at 350 to 450xc2x0 C., the fluoric polymer is adhered and fixed to the carbon fibers and the gas flow paths on the separators.
As for the catalytic layers, a mixture of a carbon powder to which a water-repelling agent of a fluoric polymer had been adhered and fixed beforehand and a platinum-carrying carbon powder has been used for forming the catalytic layers. As the above-mentioned fluoric polymer, in addition to PTFE, fluoric polymers modified with a variety of substituent groups such as perfluoromethyl group to change properties such as the glass transition point.
Furthermore, there is a solid polymer electrolyte fuel cell as a solution type fuel cell in addition to the phosphoric acid type fuel cell, and a water-repelling agent is applicable to the catalytic layers and the electrode base materials therein in the same manner.
However, fluoric polymers such as PTFE have a contact angle to water of 110xc2x0 or smaller, and thus have insufficient water-repelling properties. For example, when the output takes place at a high current density thereby to generate a large quantity of water, or when the gas flow amount is made small, the removal of generated water or condensed water is difficult depending on the operation conditions of the cell, which leads to deterioration in the cell performance. Further, such fluoric polymers exhibit insufficient adhesion to a carbon material, and there has been a problem that the fluoric polymers flow out and the water-repelling properties gradually decrease during long-term operations.
For this reason, in order to achieve a cell having a higher performance, it is desirable to use a water-repelling agent having a greater contact angle to water and high water-repelling properties. Further, in order to efficiently apply the water-repelling agent to a surface to be treated, a colloidal dispersion of the water-repelling agent needs to be thermally treated at a high temperature of 350xc2x0 C. or higher after being applied and dried. However, according to this method, it is necessary to use a water-repelling agent having a high heat resistance.
On the other hand, for the water-repelling treatment of a material having a low heat resistance, it is necessary to use a colloidal dispersion of a water-repelling agent in the applied and dried state. However, since the water-repelling agent is not adhered and fixed onto the surface to be treated, there is a possibility that the water-repelling agent comes off to cause deterioration in the water-repelling properties after long-time operations of the cell. Moreover, in such a method, it is difficult to give water-repelling treatment only to the portions intended to be treated, for example, to treat only one surface of the electrode base material.
In order to solve the problems as described above, in a fuel cell obtained by laminating unit cells, each of the unit cells including a pair of electrodes having a catalytic layer and a gas diffusion layer, an electrolyte layer disposed between the pair of electrodes, a separator having a flow path for supplying a fuel gas to one of the electrodes and a separator having a flow path for supplying an oxidant gas to the other electrode, wherein the separators are placed on the outside of the electrodes and the unit cells are stacked with the separators placed between each thereof,
the present invention provides at least the catalytic layer, the gas diffusion layer or the supply path surface is provided with water-repelling property.
It is possible to give water-repelling property to the catalytic layer, the gas diffusion layer or the flow path surface by using a water-repelling agent containing a silane compound having a hydrophobic group and a functional group, or a water-repelling agent comprising a non-polymeric compound containing a fluorine atom and a carbon atom to carry out the water-repelling treatment.
Such silane compound has preferably a hydrophobic group and a functional group, and has preferably at least either one of a hydrocarbon chain and a fluorocarbon chain on at least either one of the principal chain and the side chain of the hydrophobic group.
The silane compound is preferably a compound represented by the formula: CF3xe2x80x94(CF2)7xe2x80x94(CH2)2xe2x80x94Si(OCH3)3.
Further, as the water-repelling agent comprising a non-polymeric compound containing a fluorine atom and a carbon atom, it is preferable to use a water-repelling agent comprising a fluorinated pitch and a solvent.
The pitch has preferably an average molecular weight of 500 to 10000, and the ratio of fluorine atom to carbon atom (F/C) contained in the pitch is preferably from 1.25 to 1.65.
Moreover, the ratio of the fluorine atom to the hydrogen atom (F/H) is preferably 9 or over, and the contact angle of the fluorinated pitch to water is preferably 130xc2x0 or greater.
Furthermore, the fluorinated pitch may be synthesized by fluorinating directly a carbon type pitch or an oil type pitch.
On the other hand, the electrolyte layer preferably comprises a solid polymer membrane.
Moreover, the present invention also relates to a method for producing a fuel cell comprising stacked unit cells, each of the unit cells including a pair of electrodes having a catalytic layer and a gas diffusion layer, an electrolyte layer disposed between the pair of electrodes, a separator having a flow path for supplying a fuel gas to one electrode and a separator having a flow path for supplying an oxidant gas to the other electrode, the separators being placed on the outside of the electrodes and the unit cells being stacked with the separators placed therebetween,
the method comprising the steps of (a) applying a water-repelling agent comprising a silane compound and a solvent or a water-repelling agent comprising a fluorinated pitch and a solvent to at least a material, which constitutes the catalyst layer, the gas diffusion layer or the flow path surface, and (b) drying and removing the solvent to make the silane compound or the fluorinated pitch adhered and fixed.
Still further, the present invention relates to a method for the production of a fuel cell comprising stacked unit cells, each of the unit cells including a pair of electrodes having a catalytic layer and a gas diffusion layer, an electrolyte layer disposed between the pair of electrodes, a separator having a flow path for supplying a fuel gas to one electrode and a separator having a flow path for supplying an oxidant gas to the other electrode, the separators being placed on the outside of the electrodes and the unit cells being stacked with the separators placed between each thereof,
said method comprising a step of vapor-depositing the fluorinated pitch onto at least a material, which constitutes the catalytic layer, gas diffusion layer or the flow path surface.
In this case, it is effective to comprise a step of vapor-depositing a fluorinated pitch to the interface between the catalytic layer and the gas diffusion layer.